Implantable cardiac devices, such as pacemakers and implantable cardioverter-defibrillators (ICD's), are well-known devices that are adapted to be implanted within the body of patients and provide therapeutic stimulation to the heart to regulate heart function. Present generation devices typically include one or more leads that are adapted to be positioned adjacent the heart, circuitry for generating a therapeutic waveform, sensors which sense the function of the heart, and a processor which receives signals from the sensor and induces the waveform generation circuitry to develop and provide a waveform to the heart via the leads to regulate heart function on an as-needed basis.
An implantable cardioverter-defibrillator is a commonly used implantable cardiac device. The ICD is implanted within the body of the patient and is capable of sensing when the heart is experiencing particular forms of tachycardia requiring cardioversion or defibrillation. In particular, implantable cardioverter-defibrillators are commonly used to end ventricular fibrillation. Typically, these types of ICD's have sensors which are adapted to sense when the ventricle of the heart is fibrillating. Upon sensing the ventricular fibrillation, the processor of the ICD induces the waveform generation circuit to develop a high voltage, typically biphasic, waveform to be applied to the ventricle of the heart via the leads. Typically, the leads include an RV coil that is positioned adjacent the inner walls of the ventricle. The high voltage waveform is adapted to simultaneously depolarize substantially all of the heart cells in the ventricle so that these cells can subsequently repolarize and, preferably, function in a normal fashion. It is presently believed that fibrillation is largely characterized by these spontaneous unorganized discharging of the cells of the ventricle which results in little or no blood being pumped by the ventricle which can result in the death of the patient.
Consequently, it is desirable that ICD's be able to develop and provide the therapeutic waveform very quickly following the detection of such a cardiac event. Similarly, with other tachycardias, it is also desirable that the ICD be able to provide the therapeutic waveform very quickly after the detection of the event so as to minimize discomfort to the patient and also so as to reduce the likelihood that a comparatively mild tachycardia will develop into a more serious problem. Hence, the ability to quickly generate the waveform upon sensing the cardiac event is a serious design constraint of implantable cardiac devices like ICD's.
A further design constraint of these types of devices is that the device is typically powered by a battery implanted within the body of the patient. Consequently, it is desirable to conserve the limited energy of the battery as much as possible so as to increase the longevity of the device. Replacement of batteries can involve an invasive surgical procedure to access the implanted battery.
In order to conserve battery power, the capacitors that produce the defibrillation or cardioversion waveform are left in an uncharged state when not in use. The capacitors are then only charged when a cardiac event is detected. Leaving the capacitors in an uncharged state during periods of non-use reduces the overall drain on the battery and thereby conserves more of the limited battery energy for the generation of therapeutic waveforms during cardiac events.
However, leaving the capacitors in an uncharged state during the time intervals between the application of therapeutic waveforms to the heart can result in the capacitor degrading over time. Typically, the high voltage capacitors that are used in implantable cardiac devices, such as ICD's, are electrolytic high voltage capacitors that have an oxide dielectric. In the absence of a voltage being applied across the plates of the capacitor, the oxide dielectric can degrade over time. Subsequently, when the capacitor is charged, there can be a considerable leakage current occurring between the two plates of the capacitor as a result of the degradation of the dielectric.
This leakage current can prolong the time that it takes to charge the capacitor to the voltage necessary to produce the therapeutic waveform thereby delaying the delivery of the therapeutic waveform to the heart. In applications such as ventricular defibrillation, any delay in the application of the waveform to the heart can result in disastrous consequences for the patient. Moreover, this leakage current also requires that more energy be expended to charge the capacitor to the desire level to be able to apply the therapeutic waveform to the heart. Consequently, the leakage current can further result in excessive consumption of limited battery power thereby decreasing the longevity of the implanted device.
To address the particular problem of the capacitors of the implantable cardiac devices degrading during extended periods of non-use, implantable cardiac devices of the prior art have been adapted to periodically charge the capacitors during these extended periods of non-use. For example, it is a common practice to charge the capacitor to its maximum voltage at regular intervals, e.g., one to three months, if no shocks have been delivered during this period. While this process has the effect of reducing the degradation of the capacitor dielectric during the period of non-use, this practice is a considerable drain on the battery and can significantly reduce the total number of therapeutic waveforms that can be provided by the implanted cardiac device.
For example, for a typical capacitor used in an ICD, the capacitor will be charged during reforming maintenance to approximately 800 volts which requires the battery to provide approximately 55 joules of energy. This is a considerable expenditure of the battery's energy which significantly reduces the longevity of the battery. Moreover, the prior art systems that periodically charge the capacitors often end up charging the capacitors when dielectric has not degraded to the point where the leakage current that would occur during the generation of a therapeutic waveform would present a problem. Consequently, while periodically reforming the capacitor during periods of non-use to the capacitor's peak voltage may reduce the leakage current during therapeutic waveform generation, the reduction in leakage current is accomplished at a significant cost in terms of battery and device longevity.
Hence, there is a need for an implantable cardiac device, such as an ICD, that is capable of reforming the capacitors that provide the therapeutic waveform in a more efficient manner. To this end, there is a need for an implantable cardiac device which is capable of maintaining the capacitors in a condition such that the leakage current of the capacitor on charging is kept within acceptable tolerances without requiring a considerable expenditure of the limited energy provided by the battery of the device.